Various methods have been attempted for obtaining porous AN fibers already and, for example, there is a disclosure in the Examined Japanese Patent Publication No. 011,124/85 on the formation of water-absorptive porous AN fibers by adding cellulose acetate to AN spinning dope. However, the spinning dope to which cellulose acetate is added is with inferior stability of the dope and spinnability and is not fully satisfiable for industrial purposes. Further, heat resistance of the spun yarn is low due to a deterioration of cellulose acetate and it causes troubles during the manufacturing stages of the fiber. In addition, the quality of the product is not satisfactory.
In the Examined Japanese Patent Publication No. 042,005/86, there is a disclosure on a formation of water-absorptive porous AN fibers by adding a, nonvolatile solvent and the solvent is extracted after dry spinning. In the manufacturing steps of AN fibers either by a wet or by a dry spinning, the solvent for the spinning is usually recovered for reducing the manufacturing cost but such a means results in so much load on the recovering step of the solvent and is not so satisfactory for industrial purposes.
In the Laid-Open Japanese Patent Publication No. 025,416/72 and the Examined Japanese Patent Publication No. 008,285/73, there are disclosures in which a water-soluble compound is filled in a swollen gel tow during the manufacturing step, dried and after-treated followed by eluting the filled compound to regenerate the voids. In the Laid-Open Japanese Patent Publication No. 025,418/72, there is a disclosure in which the swollen gel tow is subjected to a wet heat treatment to make fine voids remained to impart a hygroscopicity to the AN fibers. In the conventional means of such a type, microvoids are made remained by selecting a mild drying condition to satisfy the physical properties and the dyeability of fibers. Therefore, it is extremely unstable to heating and there is a serious deterioration in quality such as a disappearance of the voids or a lowering in the shape-retaining ability of the fibrous products upon the treatment in boiling water, the steaming treatment, the ironing treatment, etc.
Moreover, such microvoids are not effective since each of the voids is apt to be present in an independent state and there is no passage which communicates the voids each other. In addition, in the Laid-Open Japanese Patent Publication No. 309,613/88, there is a disclosure in which the organic liquid with a boiling point which is not higher than Tg of the undried standard wet-spinning AN fiber and all of the water contained in the undried=fiber are substantially substituted and then dried at the temperature of not higher than the Tg of the fiber to give porous AN fibers. However, as mentioned already, there is much load on the recovery of the solvents and, in addition, the recovery of the organic liquid with low boiling point is conducted at the same time. Accordingly, that method is not industrially advantageous.
On the other hand, there is a method of introducing a cross-linking structure to the fiber for achieving the properties of the fibers such as a stability in a wet-heat, a crimp stability, a crease resistance, an improvement in elasticity, etc. For example, in the Laid-Open Japanese Patent Publication No. 005,649/75, there is a disclosure that a composite material made from a crosslinked acryl fiber and a polyamide fiber exhibits a rapid carbonation upon contacting with flames or substances of high temperature and the said composite material shows a heat-or flame-screening effect.
In the Laid-Open Japanese Patent Publication No. 012,153/77, there is a disclosure in which a polyacrylonitrile copolymer which is copolymerized with acrylamide is blended with an amino resin and, during or after the forming step of the fiber, crosslinking structures are introduced thereinto to improve the resistance to hot water and also to achieve an improvement in the fiber modulus of elasticity and a decrease in the permanent change rate. Thus, an introduction of the crosslinking structure has been attempted for improving the mechanical and physical properties of the fiber.
As mentioned hereinabove, improvements of the porous AN fibers have been mostly directed to the objects of clothing articles, nightwears and interiors which have water absorption and moisture absorption properties and, although the crosslinking structure is applied with an object of improving the mechanical and physical properties of the fiber, it is still true that, at present, thermally stable porous AN fibers having the improved property of retaining the shape of the fiber product and also of the fiber micropores and the economical manufacturing method are not obtained yet.